http://norvig.com/experiment-design.html
http://www.xibalba.demon.co.uk/jbr/ranto/
Anyway, it is very doubtful that such a claim, even if true for speakers of English learning French in England, would be true for speakers of Chinese learning Japanese in China (or the other way around) or for any number of other likely pairs of native and acquired languages.
I suspect similar effects could be found with lojban, and would love to see more research in that area. However AFAIK no such research currently exists.
I read the ranto ages ago and while it makes many good points, none of them are particularly relevant to the suggestion that the studies were faked. In fact imperfections like sexism could be advantageous if the student then proceeds to a similarly sexist European language (i.e. most of them).
http://en.wikipedia.org/wiki/Esperanto#Language_acquisition